1. Field of the Invention
The present invention relates to an inching control system in vehicles for industrial use, such as fork lifts, etc. with an automatic transmission and a mechanical clutch, which always assures the proper functioning of the inching pedal operation by automatically compensating for wear of a clutch plate.
2. Background Art
A fork lift is a specially designed vehicle to be used for cargo handling, as is well known. This vehicle is capable of forward and backward movement, as in the case of an ordinary vehicle, and also allows control of the up and down movement of the "fork" for cargo handling. The hydraulic source for driving and lifting the fork is provided power by the engine, which also supplies power for the forward and backward movement. Therefore, when a driver steps on the accelerator in order to increase the lifting speed of fork, the vehicle speed also increases. The inching operation is necessary to allow lifting of the fork while the fork lift is stopped or moving very slowly.
In conventional manual operation type fork lifts, such inching operation is carried out by stepping on a clutch pedal. For example, when the clutch is set to the OFF state (disengaged) by sufficiently stepping on the clutch pedal, the vehicle itself does not move even when an operator steps on the accelerator pedal, and the fork can be lifted up at a high speed. Moreover, it is also possible for the vehicle to be moved slowly, or "inched", by setting the clutch to a half-clutch position (partially engaged) by reducing the pressure applied to the clutch pedal. Such an inching operation requires skill on the part of the operator. To ease the burden on the operator many conventional fork lifts are provided with automatic transmissions. A fork lift with an automatic transmission, which has a mechanical clutch is explained above, is not provided with a clutch pedal, and therefore an inching pedal must be provided exclusively for inching control. FIG. 4 shows the basic construction of a vehicle of this type. In FIG. 4, 1 denotes an engine; 2, a mechanical clutch; 3, an automatic transmission; 4, a differential gear; 5, wheels; 6, a control unit (such as microcomputer, etc.) which controls clutch 2 and automatic transmission 3, etc.; 3, a clutch drive actuator; 8, a clutch stroke (position) detection sensor utilizing a potentiometer; 9, an inching pedal; 10, a sensor which detects a degree of stepping on the inching pedal; 11, an accelerator pedal; 12, a sensor which detects a degree of stepping on the accelerator pedal; 13, a hydraulic pump; 14, a hydraulic cylinder; 15, a cargo handling fork.
The automatic transmission 3 engages gears just like an ordinary manual transmission, but the shifting takes place automatically, the shifting to gears of different gearing ratios being carried out by actuator 7, which is controlled by a signal sent from the control unit 6. The controller 6 is the electronic equivalent of a shift lever in a manual transmission system. When the fork lift is put in motion, the operations of clutch 2 for OFF (i.e., disengaged), half-clutch (partially engaged) and ON (i.e., engaged) conditions are controlled by the actuator 7 and the control unit 6. Therefore, like an automatic transmission vehicle utilizing a torque converter, the automatic transmission vehicle utilizing a clutch and gear transmission as shown in FIG. 4 does not require a clutch pedal. However, such a vehicle can not realize inching control. Therefore, a pedal (inching pedal), which has a similar function as a clutch pedal, is provided in order to realize inching control.
The inching pedal 9 is not mechanically coupled with the clutch 2, and controls the clutch with a servo control system. Namely, a degree of deflection W of the inching pedal 9 is detected by the sensor 10 and input to the control unit 6 to allow a target value for servo control to be determined. The position Y of clutch 2 operated by the actuator 7 is detected by the sensor 8 and is fed-back to the control unit 6. The control unit 6 drives the actuator 7 until the actual clutch position Y reaches the target value as determined by the degree of deflection W of the inching pedal 9.
Automatic transmissions using a dry type single plate clutch and gear transmission are known, such as those employed in passenger cars. In this system, a driver is required to operate the accelerator pedal. Since the transmission is automatic, a clutch pedal and gear change lever are not provided (a lever for selecting the running mode is provided). A control circuit, including a microcomputer, senses the amount of travel of an accelerator pedal due to stepping on the accelerator pedal, the speed of the vehicle, the number of revolutions of engine (tachometer function), gear position and clutch position, etc. Moreover, when the microcomputer judges, based on such data, that the driver intends to accelerate, it opens the throttle and shifts the gears automatically when the number of rotations of engine reaches a specified value. For upshifting, the clutch is once set to OFF (clutch disengaged), the gear is shifted to the next stage through the neutral and the clutch is again smoothly coupled (clutch engaged). On the contrary, when the control unit determines that the operator intends to decelerate, it shifts down the gears.
Although the details of actual construction are different, the transmission of a fork lift is identical in principle. The difference is that a high power hydraulic pump for moving the fork is driven by the engine, and the inching pedal is provided. If a hydraulic pump is used in the automatic transmission of a passenger car, the hydraulic pump must be driven, but since a large output is not required, problems do not occur.
If the clutch 2 is of the mechanical type, the clutch will wear with continued use. This wear will affect the point at which the clutch is engaged or disengaged. A fork lift is in the half-clutch condition for much longer periods than ordinary vehicles. This is because it is stopped, started and inched repeatedly in a short period of time. Therefore, when the clutch is new, (when the amount of stepping on the inching pedal corresponds to the clutch position as shown in FIG. 5(a)), a first amount (a) of stepping on the inching pedal corresponds to clutch ON, while a second amount (b) corresponds to cluch OFF. After a period of use, however, the relation changes as shown in FIG. 5(b). Namely, the amount a' of stepping on the inching pedal corresponds to clutch ON, while the amount b' to clutch OFF, and amount (a) results in the halfclutch condition. Although differences result depending on the operating conditions to which a fork lift is subjected, such wear on the clutch may sometimes be so extreme that clutch plates must be changed daily. In this case, a large deviation in operation of the inching pedal occurs, and it is no longer possible to smoothly and precisely operate the fork lift. Particularly, a problem occurs in safe operation because there is a large deviation between operation before and after the clutch plate is exchanged.